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SubjectRe: [PATCH] sched/rt: Rework for_each_process_thread() iterations in tg_has_rt_tasks()
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Hi, Juri,

On 20.04.2018 12:25, Juri Lelli wrote:
> Hi Kirill,
>
> On 19/04/18 20:29, Kirill Tkhai wrote:
>> tg_rt_schedulable() iterates over all child task groups,
>> while tg_has_rt_tasks() iterates over all linked tasks.
>> In case of systems with big number of tasks, this may
>> take a lot of time.
>>
>> I observed hard LOCKUP on machine with 20000+ processes
>> after write to "cpu.rt_period_us" of cpu cgroup with
>> 39 children. The problem occurred because of tasklist_lock
>> is held for a long time and other processes can't do fork().
>>
>> PID: 1036268 TASK: ffff88766c310000 CPU: 36 COMMAND: "criu"
>> #0 [ffff887f7f408e48] crash_nmi_callback at ffffffff81050601
>> #1 [ffff887f7f408e58] nmi_handle at ffffffff816e0cc7
>> #2 [ffff887f7f408eb0] do_nmi at ffffffff816e0fb0
>> #3 [ffff887f7f408ef0] end_repeat_nmi at ffffffff816e00b9
>> [exception RIP: tg_rt_schedulable+463]
>> RIP: ffffffff810bf49f RSP: ffff886537ad7d50 RFLAGS: 00000202
>> RAX: 0000000000000000 RBX: 000000003b9aca00 RCX: ffff883e9cb4b1b0
>> RDX: ffff887d0be43608 RSI: ffff886537ad7dd8 RDI: ffff8840a6ad0000
>> RBP: ffff886537ad7d68 R8: ffff887d0be431b0 R9: 00000000000e7ef0
>> R10: ffff88164fc39400 R11: 0000000000023380 R12: ffffffff81ef8d00
>> R13: ffffffff810bea40 R14: 0000000000000000 R15: ffff8840a6ad0000
>> ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
>> --- <NMI exception stack> ---
>> #4 [ffff886537ad7d50] tg_rt_schedulable at ffffffff810bf49f
>> #5 [ffff886537ad7d70] walk_tg_tree_from at ffffffff810c6c91
>> #6 [ffff886537ad7dc0] tg_set_rt_bandwidth at ffffffff810c6dd0
>> #7 [ffff886537ad7e28] cpu_rt_period_write_uint at ffffffff810c6eea
>> #8 [ffff886537ad7e38] cgroup_file_write at ffffffff8111cfd3
>> #9 [ffff886537ad7ec8] vfs_write at ffffffff8121eced
>> #10 [ffff886537ad7f08] sys_write at ffffffff8121faff
>> #11 [ffff886537ad7f50] system_call_fastpath at ffffffff816e8a7d
>>
>> The patch reworks tg_has_rt_tasks() and makes it to check
>> for rt_rq::rt_nr_running instead of iteration over task list.
>> This makes the function to scale well, and its execution time
>> does not depend on number of processes in the system.
>>
>> Note, that since tasklist_lock doesn't protect a task against
>> sched_class changing, we don't introduce new races in comparison
>> to that we had before. Also, rt_rq::rt_nr_running contains queued
>> child cfs_rq in additional to queued task. Since tg_has_rt_tasks()
>
> s/cfs_/rt_/ , right?
>
>> is used in case of !runtime case:
>>
>> if (rt_bandwidth_enabled() && !runtime && tg_has_rt_tasks(tg))
>> return -EBUSY;
>>
>> the behaviour won't change. The only change is that walk_tg_tree()
>> calling tg_rt_schedulable() will break its iteration on parent cfs_rq,
>
> Ditto.
>
>> i.e. earlier.
>>
>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>> ---
>> kernel/sched/rt.c | 22 ++++++++++++----------
>> 1 file changed, 12 insertions(+), 10 deletions(-)
>>
>> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
>> index 7aef6b4e885a..601151bb9322 100644
>> --- a/kernel/sched/rt.c
>> +++ b/kernel/sched/rt.c
>> @@ -2395,10 +2395,10 @@ const struct sched_class rt_sched_class = {
>> */
>> static DEFINE_MUTEX(rt_constraints_mutex);
>>
>> -/* Must be called with tasklist_lock held */
>> static inline int tg_has_rt_tasks(struct task_group *tg)
>> {
>> - struct task_struct *g, *p;
>> + struct rt_rq *rt_rq;
>> + int cpu, ret = 0;
>>
>> /*
>> * Autogroups do not have RT tasks; see autogroup_create().
>> @@ -2406,12 +2406,18 @@ static inline int tg_has_rt_tasks(struct task_group *tg)
>> if (task_group_is_autogroup(tg))
>> return 0;
>>
>> - for_each_process_thread(g, p) {
>> - if (rt_task(p) && task_group(p) == tg)
>> - return 1;
>> + preempt_disable();
>> +
>> + for_each_online_cpu(cpu) {
>> + rt_rq = tg->rt_rq[cpu];
>> + if (READ_ONCE(rt_rq->rt_nr_running)) {
>
> Isn't this however checking against the current (dynamic) number of
> runnable tasks/groups instead of the "static" group membership (which
> shouldn't be affected by a task running state)?

Ah, you are sure. I forgot that rt_nr_running does not contain sleeping tasks.
We need to check something else here. I'll try to find another way.

Thanks,
Kirill

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